A method for the manufacture of reduced graphene oxide from kish graphite

ABSTRACT

A method for the manufacture of reduced graphene oxide from Kish graphite including the pretreatment of kish graphite, the oxidation of pre-treated kish graphite into graphene oxide and the reduction of graphene oxide into reduced graphene oxide, the reduced graphene oxide and the use of the graphene oxide.

The present invention relates to a method for the manufacture of reducedgraphene oxide from Kish graphite. In particular, reduced graphene oxidewill have applications in metal industries including steel, aluminum,stainless steel, copper, iron, copper alloys, titanium, cobalt, metalcomposite, nickel industries, for example as coating or as a coolingreagent.

BACKGROUND

Kish graphite is a byproduct generated in the steelmaking process,especially during the blast furnace process or iron making process.Indeed, Kish graphite is usually produced on the free surface of molteniron during its cooling. It comes from molten iron at 1300-1500° C.,which is cooled at a cooling rate between 0.40° C./min and 25° C./h whentransported in the torpedo car or at higher cooling rates during theladle transfer. An extensive tonnage of Kish graphite is producedannually in a steel plant.

Since Kish graphite comprises a high amount of carbon, usually above 50%by weight, it is a good candidate to produce graphene based materials.Usually, Graphene based materials include: graphene, graphene oxide,reduced graphene oxide or nanographite.

Reduced graphene oxide is composed of one or a few layers of graphenesheets containing some oxygen functional groups. Thanks to itsinteresting properties such as a high thermal conductivity and a highelectrical conductivity, reduced graphene oxide, being hydrophobic, hasmany applications as mentioned above.

Usually, reduced graphene oxide is synthesized based on Hummer Methodcomprising the following steps:

-   -   the oxidation of Kish graphite with sodium nitrate (NaNO₃),        sulfuric acid (H₂SO₄) and sodium or potassium permanganate        (KMnO₄) and    -   the reduction of graphene oxide to obtain the reduced graphene        oxide.

The patent KR101109961 discloses a method of manufacturing graphene,comprising:

-   -   a step of pretreating Kish graphite,    -   a step of manufacturing graphite oxide by oxidizing the        pretreated Kish graphite with an acid solution;    -   a step of manufacturing graphene oxide by exfoliating the        graphite oxide and    -   a step of manufacturing reduced graphene oxide by reducing the        graphene oxide with a reducing agent.

In this Korean patent, the pre-treatment of Kish graphite comprises: aflushing process, a process of purification using a chemicalpretreatment composition and a mechanical separation process (separationby size). After the process of purification, the purified Kish graphiteis separated by size, the Kish graphite having a particle size of 40mesh or less, i.e. 420 μm or less, is kept for the manufacture ofgraphene oxide.

However, the pretreatment of Kish graphite comprises 2 steps using achemical composition: the flushing step and the process of purificationstep. In the Example of KR101109961, the flushing step is performed withan aqueous solution comprising water, hydrochloric acid and nitric acid.Then, the process of purification is performed with a pretreatmentcomposition comprising a chelating agent, an iron oxide remover, asurfactant, an anionic and nonionic polymer dispersant and distilledwater. At industrial scale, two chemical treatments are difficult tomanage since a lot of chemical waste has to be treated and the stabilityof such composition is difficult to control.

Moreover, the pretreatment composition needs a long time preparation.The productivity is therefore slowed. Additionally, in the Example, thereduction of graphene oxide into reduced graphene oxide is very longsince it is performed during 24 hours.

Finally, the pre-treatment of Kish graphite including the process ofpurification using the pretreatment composition and the oxidation of thepretreated Kish graphite performed with sodium nitrate (NaNO₃), sulfuricacid (H₂SO₄) and potassium permanganate (KMnO₄) are not environmentallyfriendly. Indeed, during the pretreatment of kish graphite, a lot ofchemicals compounds are used. During the oxidation, the use of sodiumnitrate results in the formation of toxic gases such as NO₂, N₂O₄ andNH₃ which are not environmentally friendly.

The publication called “Graphene oxide and reduced graphene oxidestudied by the XDR, TEM and electron spectroscopy methods”, Journal ofelectron spectroscopy and related phenomena, vol. 195, 1 Aug. 2014,pages 145-154, discloses reduced graphene oxide denoted as FL-RGOp andFL-RGOph. FL-RGOp comprises 8.6% by weight of oxygen. FL-RGOph comprises12.1% by weight of oxygen.

However, the lateral size of both FL-RGOp and FL-RGOph are not mentionedin the publication.

The publication called “Tailoring the oxygen content of graphite andreduced graphene oxide for specific applications”, Scientific Reports,vol. 6, no. 1, 25 Feb. 2016, discloses reduced graphene oxide havingfrom 11.8% and 58.8% by weight of oxygen.

However, this publication is completely silent concerning the lateralsize of the reduced graphene oxide.

The patent application WO2018178845 discloses a Method for themanufacture of reduced graphene oxide from kish graphite comprising:

A. The provision of kish graphite,B. A pre-treatment step of said kish graphite comprising the followingsuccessive sub-steps:

-   -   i. A sieving step wherein the kish graphite is classified by        size as follows:        -   a) Kish graphite having a size below 50 μm,        -   b) Kish graphite having a size above or equal to 50 μm, the            fraction a) of kish graphite having a size below 50 μm being            removed,    -   ii. A flotation step with the fraction b) of kish graphite        having a size above or equal to 50 μm,    -   iii. An acid leaching step wherein an acid is added so that the        ratio in weight (acid amount)/(kish graphite amount) is between        0.25 and 1.0,    -   iv. Optionally, the kish graphite is washed and dried,        C. An oxidation step of the pre-treated kish-graphite obtained        after step B) in order to obtain graphene oxide and        D. A reduction of graphene oxide into reduced graphene oxide.

For example, the oxidation step C) comprises the preparation of amixture comprising the pre-treated kish-graphite, an acid and optionallysodium nitrate, the mixture being kept at a temperature below 5° C.before the addition of an oxidizing agent.

Nevertheless, when the oxidation step is performed with sodium nitrate(NaNO₃), toxic gases are produced leading to a polluting method.Moreover, the oxidation time is very long using NaNO₃.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a less pollutingmethod for the manufacture of reduced graphene oxide from Kish graphitecompared to the conventional methods. Additionally or alternately, anobject is to provide an industrial method to obtain reduced grapheneoxide having good quality in the shortest time possible.

The present invention provides a method for the manufacture of reducedgraphene oxide from kish graphite comprising: A. The provision of kishgraphite, B. A pre-treatment step of said kish graphite comprising thefollowing successive sub-steps: i. A sieving step wherein the kishgraphite is classified by size as follows: a) kish graphite having asize below 50 μm, b) kish graphite having a size above or equal to 50μm, the fraction a) of kish graphite having a size below 50 μm beingremoved, ii. a flotation step with the fraction b) of kish graphitehaving a size above or equal to 50 μm and iii. an acid leaching stepwherein an acid is added so that the ratio in weight (acid amount)/(kishgraphite amount) is between 0.25 and 1.0, C. an oxidation step of thepre-treated kish-graphite obtained after step B) in order to obtaingraphene oxide comprising the following successive sub-steps: i. thepreparation of a mixture comprising the pre-treated kish-graphite, anacid and ammonium nitrate (NH₄NO₃), the mixture being kept at atemperature below 5° C., ii. the addition of an oxidizing agent into themixture obtained in step C.i), iii. after the targeted level ofoxidation is reached, the addition of a chemical element to stop theoxidation reaction, iv. optionally, the separation of graphite oxidefrom the mixture obtained in step C.iii) and v. the exfoliation ofgraphite oxide into graphene oxide and D. a reduction of graphene oxideinto reduced graphene oxide.

The invention also provides reduced Graphene oxide comprising below 20%by weight of oxygen functional groups and having an average lateral sizebelow 30 μm comprising at least one layer sheet manufactured accordingto the method.

The following terms are defined:

-   -   Graphene oxide means one or a few layer(s) of graphene        comprising at least 45% by weight of oxygen functional groups        including ketone groups, carboxyl groups, epoxy groups and        hydroxyl groups,    -   Reduced graphene oxide means graphene oxide that has been        reduced. The reduced graphene oxide comprises one or a few        layer(s) of graphene having some oxygen functional groups and    -   A flotation step means a process for selectively separating Kish        graphite which is hydrophobic material from hydrophilic        materials.

Other characteristics and advantages of the invention will becomeapparent from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the invention, various embodiments and trials ofnon-limiting examples will be described, particularly with reference tothe following Figures:

FIG. 1 illustrates an example of one layer of reduced graphene oxideaccording to the present invention.

FIG. 2 illustrates an example of a few layers of reduced graphene oxideaccording to the present invention.

DETAILED DESCRIPTION

The invention relates to a method for the manufacture of reducedgraphene oxide from kish graphite comprising:

-   -   A. the provision of kish graphite,    -   B. a pre-treatment step of said kish graphite comprising the        following successive sub-steps:        -   i. a sieving step wherein the kish graphite is classified by            size as follows:            -   a) kish graphite having a size below 50 μm,            -   b) kish graphite having a size above or equal to 50 μm,                the fraction a) of kish graphite having a size below 50                μm being removed,        -   ii. a flotation step with the fraction b) of kish graphite            having a size above or equal to 50 μm and        -   iii. an An acid leaching step wherein an acid is added so            that the ratio in weight (acid amount)/(kish graphite            amount) is between 0.25 and 1.0,    -   C. an oxidation step of the pre-treated kish-graphite obtained        after step B) in order to obtain graphene oxide comprising the        following successive sub-steps:        -   i. the preparation of a mixture comprising the pre-treated            kish-graphite, an acid and ammonium nitrate (NH₄NO₃), the            mixture being kept at a temperature below 5° C.,        -   ii. the addition of an oxidizing agent into the mixture            obtained in step C.i),        -   iii. after the targeted level of oxidation is reached, the            addition of an element to stop the oxidation reaction,        -   iv. optionally, the separation of graphite oxide from the            mixture obtained in step C.iii) and        -   v. the exfoliation of graphite oxide into graphene oxide    -   D. a reduction of graphene oxide into reduced graphene oxide.

Without willing to be bound by any theory, it seems that the methodaccording to the present invention allows for the production of reducedgraphene oxide having good quality from high purity pre-treated Kishgraphite. Indeed, the Kish graphite obtained after step B) has a purityof at least 90%. Moreover, the method including the pre-treatment ofkish graphite, the oxidation into graphene oxide and the reduction ofgraphene oxide is easy to implement at industrial scale and is lesspolluting than methods of the prior art, in particular the one usingNaNO₃. Indeed, on the one hand, it is believed that the no toxic gasesproduced during the oxidation are N₂, O₂ and H₂O with NH₄NO₃ instead ofNO₂, N₂O₄ and NH₃ with NaNO₃. On the other hand, the amount of gasesproduced with NH₄NO₃ is higher than the one produced with NaNO₃. Thus,more gases are intercalated between the kish graphite layers so thatduring the oxidation step C.ii), KMnO₄ can easily navigate between thekish graphite layers and oxidize them. It results in a significantreduction of the oxidation time compared to NaNO₃.

Preferably, in step A), the Kish graphite is a residue of thesteelmaking process. For example, it can be found in a blast furnaceplant, in an iron making plant, in the torpedo car and during ladletransfer.

In step B.i), the sieving step can be performed with a sieving machine.

After the sieving, the fraction a) of Kish graphite having a size below50 μm is removed. Indeed, without willing to bound by any theory, it isbelieved that the kish graphite having a size below 50 μm contains avery small quantity of graphite, for example less than 10%.

Preferably in step B.ii), the flotation step is performed with aflotation reagent in an aqueous solution. For example, the flotationreagent is a frother selected from among: methyl isobutyl carbinol(MIBC), pine oil, polyglycols, xylenol, S-benzyl-S′-n-butyltrithiocarbonate, S,S′-dimethyl trithiocarbonate and S-ethyl-S′-methyltrithiocarbonate. Advantageously, the flotation step is performed usinga flotation device.

Preferably, in step B.i), the fraction a) of kish graphite having a sizebelow 55 μm is removed and in step B.ii), the fraction b) of kishgraphite has a size above or equal to 55 μm. More preferably, in stepB.i), the fraction a) of kish graphite having a size below 60 μm isremoved and wherein in step B.ii), the fraction b) of kish graphite hasa size above or equal to 60 μm.

Preferably, in steps B.i) and B.ii), the fraction b) of kish graphitehas a size below or equal to 300 μm, any fraction of kish graphitehaving a size above 300 μm being removed before step B.ii).

More preferably in steps B.i) and B.ii), the fraction b) of kishgraphite has a size below or equal to 275 μm, any fraction of kishgraphite having a size above 275 μm being removed before step B.ii).

Advantageously, in steps B.i) and B.ii), the fraction b) of kishgraphite has a size below or equal to 250 μm, any fraction of kishgraphite having a size above 250 μm being removed before step B.ii).

In step B.iii), the (acid amount)/(kish graphite amount) ratio in weightis between 0.25 and 1.0, advantageously between 0.25 and 0.9, morepreferably between 0.25 and 0.8. For example, the (acid amount)/(kishgraphite amount) ratio in weight is between 0.4 and 1.0, between 0.4 and0.9 or between 0.4 and 1. Indeed, without willing to be bound by anytheory, it seems that if the (acid amount)/(kish graphite amount) ratiois below the range of the present invention, there is a risk that thekish graphite comprises a lot of impurities. Moreover, it is believedthat if the (acid amount)/(kish graphite amount) ratio is above therange of the present invention, there is a risk that a huge amount ofchemical waste is generated.

Preferably, in step B.iii), the acid is selected among the followingelements: chloride acid, phosphoric acid, sulfuric acid, nitric acid ora mixture thereof.

Then, optionally, the kish graphite is washed and dried.

The pre-treated Kish graphite obtained after step B) of the methodaccording to the present invention has a size above or equal to 50 μm.The pre-treated Kish graphite has a high purity, i.e. at least of 90%.Moreover, the degree of crystallinity is improved compared toconventional methods allowing higher thermal and electricalconductivities and therefore higher quality.

In step C.i), the pre-treated kish graphite is mixed with an acid andammonium nitrate (NH₄NO₃). Preferably in step C.i), the acid is selectedamong the following elements: chloride acid, phosphoric acid, sulfuricacid, nitric acid or a mixture thereof. In a preferred embodiment, themixture comprises the pre-treated kish-graphite, sulfuric acid andammonium nitrate.

Preferably in step C.ii), the oxidizing agent is chosen from: potassiumpermanganate (KMnO₄), H₂O₂, O₃, H₂S₂O₈, H₂SO₅, KNO₃, NaClO or a mixturethereof. In a preferred embodiment, the oxidizing agent is potassiumpermanganate.

Then, in step C.iii), when the targeted level of oxidation is reached, achemical element is added to stop the oxidation. The targeted level ofoxidation depends on the oxidation degree of graphene oxide, i.e. havingat least 45% by weight of oxygen groups according to the presentinvention. The level of oxidation of graphene oxide can be analyzed byscanning electron microscopy (SEM), X ray diffraction spectroscopy(XRD), Transmission electron microscopy (TEM), LECO analysis and/orRaman spectroscopy over time during the oxidation.

Then, advantageously in step C.iii), the chemical element used to stopthe oxidation reaction is chosen from: an acid, non-deionized water,deionized water, H₂O₂ or a mixture thereof.

In a preferred embodiment, when at least two elements are used to stopthe reaction, they are used successively or simultaneously. Preferably,deionized water is used to stop the reaction and then H₂O₂ is used toeliminate the rest of the oxidizing agent. In another preferredembodiment, H₂O₂ is used to stop the reaction and eliminate the rest ofthe oxidizing agent. In another preferred embodiment, H₂O₂ is used tostop the reaction by this following reaction:

2KMnO₄+3H₂O₂=2MnO₂+3O₂+2KOH+2H₂O.

Then, to eliminate MnO₂, an acid can be used. For example, HCl is addedto the mixture so that the following reaction happens:

MnO₂+2HCl=MnCl₂ (soluble in water)+H₂O.

Without willing to be bound by any theory, it seems that when theelement to stop the reaction is added into the mixture, there is a riskthat this addition is too exothermic resulting in explosion orsplashing. Thus, preferably in step C.iii), the element used to stop thereaction is slowly added into the mixture obtained in step C.ii). Morepreferably, the mixture obtained in step C.ii) is gradually pumped intothe element used to stop the oxidation reaction. For example, themixture obtained in step C.ii) is gradually pumped into deionized waterto stop the reaction.

In step C.iv), graphite oxide is separated from the mixture obtained instep C.iii). Preferably, the graphite oxide is separated bycentrifugation, by decantation or filtration.

Then, optionally, the graphite oxide is washed. For example, thegraphite oxide is washed with an element chosen from among: deionizedwater, non-deionized water, an acid or a mixture thereof. For example,the acid is selected among the following elements: chloride acid,phosphoric acid, sulfuric acid, nitride acid or a mixture thereof.

After, optionally, the graphite oxide is dried, for example with air orat high temperature in the vacuum condition.

Preferably in step C.v), the exfoliation is performed by usingultrasound or thermal exfoliation. Preferably, the mixture obtained instep C.iii) is exfoliated into one or a few layers of graphene oxide.

After step C), Graphene oxide comprising at least 45% by weight ofoxygen functional groups and having an average lateral size between 5and 50 μm, preferably between 10 and 40 μm and more preferably between10 and 30 μm comprising at least one layer sheet is obtained.

Then, preferably, in step D), the graphene oxide is partially orcompletely reduced in order to obtain a reduced graphene oxide havingfrom 0.4% to 25% by weight, more preferably from 1 to 20% of oxygengroups.

Preferably, step D) comprises the following sub-steps:

-   -   i. The reduction of graphene oxide with a reducing agent,    -   ii. agitation of the mixture obtained in step D.i),    -   iii. optionally, the washing of the reduced graphene oxide and    -   iv. optionally, the drying of the reduced graphene oxide.

In step D.i), preferably, the reducing agent is chosen from: acidascorbic; urea; hydrazine hydrate; alkaline solution such as NaOH orKOH; phenols such as gallic acid, tannin acid, dopamine or teapolyphenol; alcohols such as methyl alcohol, ethyl alcohol or isopropylalcohol; glycine; sodium citrate or sodium borohydride. More preferably,the reducing agent is acid ascorbic since the ascorbic acid is moreenvironmentally friendly.

Advantageously, in step D.ii), the mixture is kept at a temperaturebetween 50 and 120° C., more preferably between 60 and 95° C. andadvantageously between 80 and 95° C. Preferably, the agitation isperformed during less 24 hours, more preferably during less than 15hours and advantageously during 1 to 10 hours.

By applying the method according of the present invention, reducedgraphene oxide comprising below 20% by weight of oxygen functionalgroups and having an average lateral size below 30 μm preferably below20 μm and more preferably below 10 μm comprising at least one layersheet is obtained.

FIG. 1 illustrates an example of one layer of reduced graphene oxideaccording to the present invention. The lateral size means the highestlength of the layer through the X axis, the thickness means the heightof the layer through the Z axis and the width of the nanoplatelet isillustrated through the Y axis.

FIG. 2 illustrates an example of a few layers of reduced graphene oxideaccording to the present invention. The lateral size means the highestlength of the layer through the X axis, the thickness means the heightof the layer through the Z axis and the width of the nanoplatelet isillustrated through the Y axis.

Preferably, reduced graphene oxide is deposited on metallic substratesteel to improve some properties such as corrosion resistance of ametallic substrate.

In another preferred embodiment, reduced graphene oxide is used ascooling reagent. Indeed, graphene oxide can be added to a cooling fluid.Preferably, the cooling fluid can be chosen from among: water, ethyleneglycol, ethanol, oil, methanol, silicone, propylene glycol, alkylatedaromatics, liquid Ga, liquid In, liquid Sn, potassium formate and amixture thereof. In this embodiment, the cooling fluid be used to cooldown a metallic substrate. For example, the metallic substrate isselected from among: aluminum, stainless steel, copper, iron, copperalloys, titanium, cobalt, metal composite, nickel.

The invention will now be explained in trials carried out forinformation only. They are not limiting.

Examples

Trials 1, 2 and 3 were prepared by providing Kish graphite fromsteelmaking plant. Then, Kish graphite was sieved to be classified bysize as follows:

a) Kish graphite having a size below <63 μm and

-   -   b) Kish graphite having a size above or equal to 63 μm.        The fraction a) of Kish graphite having a size below 63 μm was        removed.

For Trials 1 and 2, a flotation step with the fraction b) of Kishgraphite having a size above or equal to 63 μm was performed. Theflotation step was performed with a Humboldt Wedag flotation machinewith MIBC as frother. The following conditions were applied:

-   -   Cell volume (I): 2,    -   Rotor speed (rpm): 2000,    -   Solid concentration (%): 5-10,    -   Frother, type: MIBC,    -   Frother, addition (g/T): 40,    -   Conditioning time (s): 10 and    -   Water conditions: natural pH, room-temperature.

All Trials were then leached with the hydrochloric acid in aqueoussolution. Trials were then washed with deionized water and dried in airat 90° C.

After, Trial 1 was mixed with ammonium nitrate and sulfuric acid whileTrials 2 and 3 were mixed with sodium nitrate and sulfuric acid in anice-bath. Potassium permanganate was slowly added into Trials 1 to 3.Then, mixtures were transferred into water bath and kept at 35° C. tooxidize the Kish graphite.

After the oxidation, Trials were gradually pumped into deionized water.

For Trial 1, the heat was removed and H₂O₂ in aqueous solution was addeduntil there was no gas producing. MnO₂ was produced. Then, HCl was addedto the mixture to eliminate MnO₂.

For Trials 2 and 3, After stopping the oxidation reaction, the heat wasremoved and H₂O₂ in aqueous solution was added until there was no gasproducing and mixtures were stirred to eliminate the rest of H₂O₂.

Then, for all Trials, Graphite oxide was separated from the mixture bydecantation. They were exfoliated using ultrasound in order to obtainone or two layer(s) of graphene oxide. Finally, graphene oxide wasseparated from the mixture by centrifugation, washed with water anddried with air.

L-ascorbic acid were mixed with aqueous solutions of Trials 1 to 3. Thereaction mixtures were agitated at 90° C. to reduce the graphene oxidesheets. All Trials were then washed and dried to obtain reduced grapheneoxide powder.

Graphene oxide and Reduced graphene oxide were analyzed by scanningelectron microscopy (SEM), X ray diffraction spectroscopy (XRD),Transmission electron microscopy (TEM), LECO analysis and Ramanspectroscopy.

Trials 2 and 3 correspond respectively to Trials 1 and 2 ofWO2018178845. Table 1 shows the results obtained.

Trial 2 Trial 3 Method Trial 1 * (Trial 1 of WO2018178845) (Trial 2 ofWO2018178845) Origin of Kish graphite Steelmaking plant Steelmakingplant Steelmaking plant Pre- Sieving step Done, Kish graphite having aDone, Kish graphite having a size Done, Kish graphite having a sizetreatment size above or equal to 63 μm above or equal to 63 μm keptabove or equal to 63 μm kept of Kish kept graphite Flotation step DoneDone Not done Acid leaching Done with HCl, (the acid Done with HCl, (theacid Done with HCl, (the acid step amount)/(kish graphite amount)/(kishgraphite amount) amount)/(kish graphite amount) amount) ratio in weightis of ratio in weight is of 0.78 ratio in weight is of 1.26 0.78Pre-treated kish graphite 95% 95% 74.9% purity Oxidation preparationDone with H₂SO₄ and NH₄NO₃ Done with H₂SO₄ and NH₄NO₃ Done with H2SO₄and NH₄NO₃ step of the mixture Gases N2, O2 and H2O NO2, N2O4 and NH3NO2, N2O4 and NH3 produced Addition of KMnO₄ KMnO₄ KMnO₄ an oxidizingagent Oxidation 1 h 30 min 3 hours 3 hours time Element to Waterfollowed by H₂O₂ Water followed by H₂O₂ Water followed by H₂O₂ stop thereaction Exfoliation Ultrasound Ultrasound Ultrasound Graphene oxideGraphene oxide comprising Graphene oxide comprising 40% of Grapheneoxide comprising 30% of 49% of oxygen groups and oxygen groups andhaving an oxygen groups and having an having an average Lateral sizeaverage Lateral size from 20 to 35 average lateral size from 20 to 35from 10 to 20 μm with purity of μm with purity of 99.5% μm with purityof 99.0% 99.5% Reduction step Done with acid ascorbic during Done withacid ascorbic during 3 Done with acid ascorbic 3 hours hours during 3hours Reduced graphene oxide Reduced Graphene oxide Reduced Grapheneoxide Reduced Graphene oxide comprising 17% of oxygen comprising 15% ofoxygen comprising 15% of oxygen groups groups and having an averagegroups and having an average and having an average Lateral size Lateralsize from 15 to 30 μm Lateral size from 20 to 30 μm with from 20 to 30μm with purity of 99.5% and an average purity of 99.5% and an average99.0% and an average thickness thickness of 1-6 nm thickness of 1-6 nmof 1-6 nm * according to the present invention

The method of Trial 1 is more environmentally friendly than the methodused for Trials 2 and 3. Moreover, the oxidation time with the method ofTrial 1 is divided by two. Finally, the reduced graphene oxide obtainedwith Trial 1 has a high quality.

1-17. (canceled)
 18. A method for manufacture of reduced graphene oxidefrom kish graphite comprising: A. providing kish graphite, B.pre-treating the kish graphite including the following successivesub-steps: i. a sieving step wherein the kish graphite is classified bysize as follows: a) kish graphite having a size below 50 μm, b) kishgraphite having a size above or equal to 50 μm,  the fraction a) of kishgraphite having a size below 50 μm being removed, ii. a flotation stepwith the fraction b) of kish graphite having a size above or equal to 50μm and iii. an acid leaching step wherein an acid is added so that aratio in weight (acid amount)/(kish graphite amount) is between 0.25 and1.0, C. oxidizing the pre-treated kish-graphite obtained after step B)in order to obtain graphene oxide including the following successivesub-steps: i. preparation of a mixture including the pre-treatedkish-graphite, an acid and ammonium nitrate (NH₄NO₃), the mixture beingkept at a temperature below 5° C., ii. addition of an oxidizing agentinto the mixture obtained in step C.i), iii. after a targeted level ofoxidation is reached, addition of a chemical element to stop theoxidation reaction, iv. optionally, separation of graphite oxide fromthe mixture obtained in step C.iii) and v. exfoliation of the graphiteoxide from the mixture obtained in step C.iii) into graphene oxide, andD. reducing of the graphene oxide into reduced graphene oxide.
 19. Themethod as recited in claim 18 wherein in step B.i), a fraction a) ofkish graphite having a size below 55 μm is removed and in step B.ii), afraction b) of kish graphite has a size above or equal to 55 μm.
 20. Themethod as recited in claim 19 wherein in steps B.i) and B.ii), thefraction b) of kish graphite has a size below or equal to 300 μm, anyfraction of kish graphite having a size above 300 μm being removedbefore step B.ii)
 21. The method as recited in claim 18 wherein in stepB.iii), the acid amount/kish graphite amount ratio in weight is between0.25 and 0.9.
 22. The method as recited in claim 18 wherein in stepB.iii), the acid is selected from the group consisting of the followingelements: chloride acid, phosphoric acid, sulfuric acid, nitric acid anda mixture thereof.
 23. The method as recited in claim 18 wherein in stepC.ii), the oxidizing agent is chosen from the group consisting of:potassium permanganate (KMnO₄), H₂O₂, O₃, H₂S₂O₈, H₂SO₅, KNO₃, NaClO anda mixture thereof.
 24. The method as recited in claim 18 wherein in stepC.iii), the chemical element used to stop the oxidation reaction ischosen from the group consisting of: an acid, non-deionized water,deionized water, H₂O₂ and a mixture thereof.
 25. The method as recitedin claim 24 wherein the chemical element and a further chemical elementchosen from the group are used to stop the oxidation reaction, theelement and the further chemical element being used successively. 26.The method as recited in claim 24 wherein the chemical element and afurther chemical element chosen from the group are used to stop theoxidation reaction, the element and the further chemical element beingused simultaneously.
 27. The method as recited in claim 18 wherein instep C.iii), the mixture obtained in step C.ii) is gradually pumped intothe element used to stop the oxidation reaction.
 28. The method asrecited in claim 18 wherein in step C.vii), the exfoliation is performedby using ultrasound or thermal exfoliation.
 29. The method as recited inclaim 18 wherein step C.iv) is performed and the graphite oxide isseparated by centrifugation, by decantation or filtration.
 30. Themethod as recited in claim 18 wherein in step C.i), the acid is selectedfrom the group consisting of the following elements: chloride acid,phosphoric acid, sulfuric acid, nitric acid and a mixture thereof. 31.The method as recited in claim 18 wherein in step D) includes thefollowing sub-steps: i. reduction of graphene oxide to reduced grapheneoxide with a reducing agent, ii. agitation of the mixture obtained instep D.i), iii. optionally, washing of the reduced graphene oxide andiv. optionally, drying of the reduced graphene oxide.
 31. The method asrecited in claim 31 wherein in step D.i), the reducing agent is chosenfrom the group consisting of: acid ascorbic; urea; hydrazine hydrate;alkaline solution such as NaOH or KOH; phenols such as gallic acid,tannin acid, dopamine or tea polyphenol; alcohols such as methylalcohol, ethyl alcohol or isopropyl alcohol; glycine; sodium citrate andsodium borohydride.
 32. The method as recited in claim 21 wherein instep D.ii), the mixture is kept at a temperature between 50 and 120° C.33. The method as recited in claim 21 wherein in step D.ii), theagitation is performed during less than 24 hours.
 34. Reduced Grapheneoxide comprising below 20% by weight of oxygen functional groups andhaving an average lateral size below 30 μm including at least one layersheet obtainable from the method as recited in claim 18.